Recently, interests in energy storage technologies have been increasingly grown. While the application of the energy storage technologies is expanded to mobile phones, camcorders, notebook PCs, and even to electric vehicles, efforts for research and development of electrochemical devices have been materialized.
The development of rechargeable secondary batteries among these electrochemical devices has become the center of attention. Recently, in the development of the secondary batteries, research into the development of the design of novel electrode and battery has been conducted in order to improve capacity density and specific energy.
Among the currently used secondary batteries, lithium secondary batteries, developed in the early 1990's, are spotlighted because the lithium secondary batteries may have higher operating voltage and significantly higher energy density than conventional batteries, such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries, using aqueous electrolytes. However, the lithium secondary batteries may have limitations in that fire and explosion may occur due to the use of organic electrolytes and the manufacture thereof may be difficult. Accordingly, lithium-ion polymer batteries for improving the limitations of the lithium secondary batteries have been developed. However, the capacities thereof may be relatively lower than those of the lithium secondary batteries.
During initial charge of a lithium secondary battery, lithium ions generated from a positive electrode active material, such as a lithium metal oxide, move to a negative electrode active material, such as graphite, to be intercalated into the negative electrode active material. In this case, since the lithium ions are highly reactive, the lithium ions react with an electrolyte and carbon constituting the negative electrode active material on a surface of the negative electrode active material to form compounds such as Li2CO3, Li2O, or LiOH. These compounds form a kind of stable film (solid electrolyte interface, SEI) on the surface of the negative electrode active material. The film formed on the surface of the negative electrode active material may only pass the lithium ions by acting as an ion tunnel and may prevent the destruction of the structure of a negative electrode caused by the intercalation of organic solvent molecules having a high molecular weight, which move with the lithium ions in the electrolyte, into the negative electrode active material. Also, since the film may prevent the contact between the negative electrode active material and the electrolyte, the decomposition of the electrolyte may not occur and the amount of the lithium ions in the electrolyte may be reversibly maintained. Thus, stable charge and discharge may be maintained.
However, with respect to the lithium secondary battery, since the carbon material may be desorbed from an electron transfer pathway due to the changes in lattice parameter of carbon and the generation of gas caused by the decomposition of the solvent as the charge and discharge proceed, the capacity thereof may be reduced. Thus, in order to address these limitations, there is a continuous need to develop a method of improving initial capacity and power of the battery.